1. Field of the Invention
The invention relates to a method for the electrolytic deposition of Group IB and VIII metals of the periodic table and in addition, Zn, Cd, Sn and Pb on a solid electrolyte. The invention relates further to an arrangement for conducting the method as well as to a product made thereby.
2. Description of the Prior Art
Of the methods for the coating of electronically non-conducting materials the best known are the so-called currentless deposition methods (e.g. F. A. Lowenheim ed., "Modern Electroplating," Bd. Edition, Wiley-Interscience, New York, 1974, Chapter 28, "Plating of Nonconductors," pp. 636-652, as well as the literature references and patents cited on pages 652-655). In these methods the substrate to be coated is immersed in a bath containing the metal ion concerned in simple or complex form, to which a reducing agent is gradually added. As a rule still other additives are mixed into the bath as stabilizers and accelerators. The substrate surface must be so constituted that it catalytically accelerates the reduction of the metal ion. Surfaces of electronic nonconductors must, before immersion in the bath, be activated by a suitable preliminary treatment (e.g. by dipping in PdCl.sub.2 or SnCl.sub.2 solutions).
The cited methods are involved and difficult and the yield of metal is often uneconomical, especially when costly metals (e.g. noble metals) are to be deposited. The baths must be tended and controlled extremely precisely with respect to pH value, concentrations of the reacting substances, impurities, temperature and other operating parameters. Often it is not possible to achieve an economically supportable optimization of all parameters, so that the output lags far behind the set goal. Moreover, solid electrolyte substrates coated via currentless deposition exhibit only small transverse electrical conductivity, and the metal layers are not adhesive enough in subsequent use (charging the electrolysis cell).
Industry places very demanding conditions on metal coatings of ionic conducting materials like solid electrolytes and ion-exchange membranes. Along with good adherence to the substrate, the metal layer should have the greatest possible specific surface area and a high transverse electronic conductivity while simultaneously guaranteeing good permeability (porosity) to liquids and gases. On the other hand, the amount of metal to be deposited per unit area of the substrate should be kept as low as possible.